Motor control system



Jan. 4, 1938. B. w. JONES MOTOR CONTROL SYSTEM Filed Dec.

26 Inventor":

Benjamin Wdones by His Attorney.

Patented Jan. 4, 1938 UNITED STATES PATENT OFFICE MOTOR CONTROL SYSTEM New York Application December 31, 1936, Serial No. 118,479

Claims.

This invention relates to control systems, more particularly to systems for controlling the operations of electric motors, and it has for an object the provision of a simple, reliable and im- 6 proved system of this character.

More specifically, the invention relates to systems for controlling the starting and accelerating of a motor, and a more specific object of the invention is the provision of means utilizing en- 10 tirely standard electromagnetic switching de vices for efifecting a shunting of the accelerating resistor in steps separated by time intervals, and varying these time intervals in accordance with the rate at which the motor comes up to speed. In other words, the invention provides means for accelerating a lightly loaded motor at a more rapid rate than a heavily loaded motor.

In carrying the invention into effect in one form thereof, means are provided for shunting one of the resistors in the motor circuit, and an inductive type time delay relay having a coil connected to be responsive to the shunting of this resistor efiects the shunting of a second resistor after an interval of time. One terminal of the coil is connected to a terminal of the motor armature, and in order to vary the time interval of the relay in accordance with the rate at which the motor comes up to speed, a connection is provided from the opposite terminal of the coil to the opposite terminal of the motor armature, and a resistor of a relatively high resistance is included in this connection.

For a better and a more complete understanding of the invention, reference should now be had to the following specification and to the accom+ panying drawing in which Fig. l is a simple, schematic diagram of an embodiment of the invention, and Fig. 2 is a further simplified diagram in across the line form; of certain of the elements and connections of the system of Fig. 1.

Referring now to the drawing, the armature of the motor I6 having a series field winding II, is arranged to be connected to a direct-current source of supply I2, I3 for rotation in either di-' g5 motion by means of reversing or directional contactors H, l5 for the forward direction and I6, H for the reverse direction. A suitable current limiting device is included in the armature circuit for protecting the motor during starting. Such current limiting device is illustrated as comprising a plurality of resistors l8, l9 and 20. Resistor i8 is utilized to limit the armature current during plugging operation of the motor and is therefore referred to as the plugging resistor. Resistors l9 and 20 serve to control the motor current during acceleration and are therefore referred to as accelerating resistors. The plugging resistor I8 is controlled by means of plugging contactor 2 I, and similarly, accelerating resistors I9 and are respectively controlled by 5 accelerating contactors 22 and 23. A line contactor 24 is provided for completely disconnecting the motor from the source of supply when a disconnecting switch is opened.

A pair of electromagnetic switches or relays and 26 are provided for controlling the plugging resistor contactor l8, and similar relays 21 and 28 are provided for controlling the accelerating contactors 22 and 23 respectively. These contactors are under the control of a multipoint reversing type master switch 29. The winding of the relay 2'! is connected in parallel with the plugging resistor l8 so as to be responsive to the voltage drop across the resistor, and the winding of the relay 2,6 is connected in parallel with 2 the plugging resistor l8 and the first accelerating resistor Hi. This is for the purpose of obtaining a proper sequence in the action of the relays.

The winding of the relay 25 is arranged to be connected across the motor armature and the plugging resistor l8 for one direction of motor operation, and the winding of the relay 26 is similarly connected for the opposite direction of motor operation, It will be observed that these plugging relays 25 and 26 are normally open devices, that is, the switch contacts controlled by these relays are biased by means of springs to their open positions. vRelays 21 and 28, on the other hand, are normally closed. devices, since their controlling contacts are normally in the closed position when the operating windings of these relays are deenergized. I

The relays 25 to 26 inclusive are preferably of identical construction with the exception that the contacts of the relays 25 and 26 are normally in their open position, whereas the relays 21 and 28 have their contacts normally in the closed position. Each of these relays has a winding having a sufficient number of turns so that with the armature of the relayin the attracted position, the magnetic flux of the electromagnet is considerably in excess of the value required to hold the armature in the attracted, position. This feature is advantageous, since the timing action obtained by the means of the relay will be substantially unaffected by variations in the voltage of the source of supply and variations of the heating of the winding. .Relays' of this type are standard devices available on the market, 5

and their specific structure constitutes no part of the present invention.

To provide for varying the time interval in the drop-out operation of the relays 22' and 22 in accordance with the rate at which the motor is coming up to speed, a connection is provided from each of the operating coils of the relays 2E and 28 to one terminal of the armature of the motor it, and a resistor having a relatively high value of resistance is included in each of these connections.

It is to be noted that one terminal of the operat, ing coil of relay El is permanently connected to the common point between the armatureof the motor 1% and the plugging resistor l3, and therefore, the opposite terminal of the'operatin'g coil 2? is connected through the highresistance 30 to the opposite terminal of the armature of motor l3. Similarly, it is to be noted that one terminal of the operating coil of relay 28 is connected to the common point between one terminal of the motor armature and the plugging resistor l8, and therefore, the opposite terminal of the coil of relay 28 is connected through high resistance 3| to the opposite terminal of the motor armature. This latter connection may be made directly through the resistance 3| to the opposite terminal of the motor armature, or through resistances 3| and 38 to the motor armature, as shown in the drawing.

In addition, to the resistors 38 and 3|, additional resistors and 33 are respectively'connected in the circuit of the operating coils 21 and 28.

The resistor 32 serves to make possible a wide variation in the voltage of the point of its connection to the operating coil of relay 2?, and similarly, the resistor 33 serves to make possible a wide variation in the voltage of the point of its connection to the coil of relay 28. By connecting these two points of variable voltage to the armature terminal opposite to that to which the terminals of the coils of relays 2'? and 28 are connected, it is possible so to change the voltages applied to the coils of'relays 21 and 28 as a func tion of the countervoltage of the motor that the current in the coils of relays 2'! and 23 will be reversed. This will change the drop-out time of these relays in proportion to the rapidity with which their currents are reversed.

The invention is not limited to any particular ohmic value of the resistors 30, 3|, 32 and 33, but typical values of these'resistors which have been found to give good results on a 250-volt circuit are as follows:

' Ohms Resistor 33 5000 Resistor 3| i"; Resistor 32 1000 Resistor 33 1000 As thus'constructed and arranged, the operation of the-invention is as follows: The disconnecting switches 34 and 35 areclosed. The closing of switch 34 completes an energizing circidt for the operating coil of line switch 2 This circuit is traced from the positive supply line 52 through switch 34, conductors 36 and segments 38, '39 and 48 of the master switch 25. conductor 4|, operating coil of line switch 24, conductors 32 and 43 to the opposite side of the supply source. Line switch 24 closes in response to energization but produces no elfect on the motor at this time, because the master switch is in the on or neutral position. 'The closing of the line'switch 24 establishes a holding circuit for its'operating coil so that the master switch may be moved from the 01f position to an' operating position and the line switch still remain energized and closed.

If the master switch 29 is now moved from the off position toward the left to its first operating position, the forward directional contactors l4 and 15 will be energized through a circuit extending from the positive supply line |2 through switch 34, conductor 35, closed interlock of line contactor 2i, conductor segments 49 and 45 of the master switch, conductor 46, operating coils of contactors i4 and |5 in parallel and thence by conductor 43V to the negative side of the supply source. Conta'ctors i4 and |5 close in response to energization and connect the armature of motor iii to the supply source with the resistors I8, I 9 and 20 in series relationship therewith.

It will be noted that as soon as the motor is connected to the supply circuit, the windings of relays 2'! and 28 are immediately energized to open their respective. contacts.-. This is because the winding of relay 2? is energized in multiple relation with the plugging resistor l8, and the winding of relay 28 is energized in multiple relation with the plugging resistor H3 and accelerating resistor l9. Since these relays are energized, the contacts controlled thereby are magnetically held in their respective open positions so that the accelerating contactors 22 and 23 will not be energized at this time.

The relay 25, however, will be energized through the lower auxiliary contacts of forward contactor 4, thence through conductor 41, operating coil of relay 25, conductor 48, to the point of connection between plugging resistor l8 and accelerating resistor l9. Thus, the operating coil of relay 25 is now connected across the armature of motor l6 and the plugging resistor |8. Since the motor is being started from rest, the motor armature will not have developed a 'countervoltage until the motor starts, so that as a practical matter, the winding of the relay 25 will be energized by the voltage drop across the plugging resistor Hi. This will be of suflicient value to energize the relay 25 and to cause it to close its contacts. This will produce no effect at this time until the master switch 29 is thrown toward the left to its second operative position, but when the master switch is thrown to the second position, the plugging contactor 2| is energized to close immediately. The circuit for the operating coil of plugging contactor 2| is traced from the positive supply conductor, through auxiliary contacts of line switch 24, segments 40 and 45 of the master switch, conductor 46, contacts 25a bridged by the movable contact member 25b, conductor 49, finger 58, segments 5| and 52 of the master switch, finger 53, conductor 54, and operating coil of plugging contactor 2| to the negative side of the supply source. In response to energization, conductor 2i closes its contacts to short-circuit plugging resistor l8. 'Although' the operation of the master switch 29 from its off position to its operating position'is described as taking place in steps, it will be understood that in practice, operators usually throw the master switch quickly from the off position to the full running position. However, in order to visualize the sequence of operations, it is necessary here to consider the operation of the master switch as taking place in successive steps. When the plugging contactor 2| closes its main contacts, it also short-circuits the winding of the relay 21 which is connected across the plugging resistor l8 through the resistor 32. Prior to the short-circuiting of the resistor l8, the direction of current through the operating coil of relay 21 is represented by the arrow above the coil in Fig. 2.

As previously explained, the short-circuiting of the winding of relay 21 does not efiect the immediate release of the relay armature, because it takes an appreciable interval of time for the flux of the magnetic structure of the relay to decrease on account of the inductive action of the relay winding, and thus an appreciable interval of time elapses before the biasing spring of the relay armature overpowers the magnetic pull on the relay armature and eiTects the closing of the relay contacts. In this connection, it is to be remembered that when the operating winding of the relay 21 is short-circuited, the inductance of the coil tends to keep the current flowing in the same direction as that in which it was flowing prior to short-circuiting. As longas this current is of sufficient magnitude to overpower the pull of the spring, the contacts of relay 21 will be maintained open. As previously pointed out, the left-hand terminal of the winding of relay 21 in Fig. 2 is connected to the point of connection between the plugging resistor l8 and the armature of motor I0, and the opposite terminal of winding of relay 21 is connected through the resistor 30 to the opposite or positive terminal of the armature l0. Thus, prior to short-circuiting of the plugging resistor I8,-the voltage drop between the positive terminal of the motor and the connection point between resistors l8 and I9 is taken up in the resistors 30 and 32, and there is no'tendenoy to reverse the direction of current flow through the winding of relay 21. The speed of the motor I0 is practically zero at the instant the plugging resistor I8 is short-circuited, the countervoltage of the motor In is practically zero; and thus the voltage of the negative motor terminal 56 is substantially the same as the positive motor terminal except for the IR drop through the armature. However, as the armature of motor l8 begins to accelerate and comes up to speed, the voltage of the terminal 56 becomes negative as the speed of the motor increases, and thus the countervoltage of the motor tends to reverse the direction of current flow in the coil 21. At this instant, the countervoltage tends to cause current to flow from the positive terminal 55 of the motor through auxiliary contacts '43, of forward contactor l4, resistor 30, through coil of contactor 21 in the direction represented by the dotted arrow in Fig. 2 to the negative terminal of the motor 56 and thence through accelerating resistors l9 and 28 to the negative side of the supply source. Thus it will be noted that as the motor comes up to speed, and the terminal 56 of the motor becomes increasingly negative, the current flow through the operating coil of relay 21 is reversed at a rate depending upon the rate at which the motor is accelerating. This changes the drop-out time of the relay 21, i. e., it decreases the drop-out time in accordance with the rate of acceleration of the motor. In other words, the faster the motor comes up to speed, the shorter the drop-out time of the relay 21. Thus, an interval of time after short-circuiting of the plugging resistor 18, which interval of time is varied in accordance with the rate of acceleration of the motor [0, the contacts of the relay 21 are closed to complete an energizing circuit for the accelerating contactor 22, provided the master switch 29 has been operated to its third operative position. This energizing circuit is traced from the positive supply lines to the power segments 40 of the master switch 28 as before, segment 45,

conductor 46, contacts of relay 25, conductor 49, finger 58 and segment 5| of the master switch,

segment 51, finger 58, conductor 59, operating coil of contactor 22, conductor 68, contacts of relay 21 in the closed position thereof, and thence by conductor Bl to the negative side of the supply source. Contactor 22 will thus be energized to close and to short-circuit the accelerating resis tor IS an interval of time after the short-circuiting of the plugging resistor I8, and as previously mentioned, this interval of time varies with the rate at which the motor is accelerating.

The closing of the contactor 22 also short-circuits the winding of the relay 28 so that after an interval of time, the spring associated with the armature of the relay 28 will overpower the magnetic attraction of the armature of the relay, and the relay contacts will close. The winding of contactor 23 is thus energized through a portion of the circuit previously traced for the winding of contactor 2| as far as the conductor 59 and thence through the coil of contactor 23, conductor 62 and contacts of relay 28 to conductor 6| and the negative side of the supply source. As previously stated, the right-hand or negative terminal of the winding of relay 28 is connected back through resistors 3| and 30 to the positive terminal 55 of the motor armature, and thus when the contactor 22 closed to short-circuit the accelerating resistor IS, the countervoltage of the motor became effective to reverse the direction of the current flow through the winding of relay 28 and thereby to vary its drop-out time in l the motor ID, the contacts of the relay 28 are closed, the operating coil of contactor 23 is energized and the contacts of contactor 23 closed to short-circuit the final accelerating resistor 28. The motor I0 is thus accelerated from rest to full running speed as a function of time and the countervoltage of the motor. In other words, if the motor is heavily loaded, then the time required to accelerate is longer than if the motor is less heavily loaded and coming up to speed with less effort.

The motor is stopped by operating the master switch 29 to its off position.

Operation of the motor in the reverse direction is effected by operating the master switch 29 in the reverse direction. This operation is similar to that described for operation in the forward direction, and repetition is therefore unnecessary.

Although in accordance with the provisions of the patent statutes, this invention is described as embodied in concrete form, it will be understood that the specific elements and their connections in the system are merely illustrative and that the invention is not limited thereto, since alterations and modifications will readily suggest themselves to persons skilled in the art without departing from the true spirit of this invention or from the scope of the annexed claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A control system for an electric motor comprising in combination a plurality of current limiting devices in the armature circuit of the motor, means for shunting one of said devices, means responsive to the shunting of said device for controlling shunting another of said current limiting devices after an interval of time comprising an inductive time delay device having an operatingcoil connected to said first current limiting device, and means responsive to the countervoltage of said motor for varying the energization of said coil thereby to vary said time interval.

2. A control system for an electric motor comprising in combination a plurality of current limiting resistors connected in the armature circuit of said motor, means for shunting one of said resistors, means responsive to the shunting of said resistor for controlling the shunting of another of said resistors after an interval of time comprising an inductive time delay device having an operating coil connected to said first resistor, and means responsive to the countervoltage of said motor for reversing the current through said coil at a rate dependent upon the rate of acceleration of said motor.

3. A motor starter comprising a plurality of resistors connected between one terminal of the motor and one side of a supply source, means for shunting one of said resistors, means responsive to the shunting of said resistorrfor controlling the shunting of a second resistor after an interval of time comprising an inductive time delay relay having its operating coil connected across said first resistor and means responsive to the countervoltage of said motor for reversing the current in said coil thereby to vary said time interval in accordance with the rate of acceleration of said motor comprising a connection from the other terminal of said armature to said coil, and a resistor in said connection.

4. A motor starter comprising in combination a plurality of resistors connected in the motor circuit, means for shunting one of said resistors, means responsive to the shunting of said resistor for controlling the shunting of a second of said resistors after an interval of time comprising an inductive time delay relay having one terminal connected to an armature terminal of said motor and having its opposite terminal connected to a point of said first resistor, and means for varying said time interval in accordance with the rate of acceleration of said motor comprising a connection from the opposite armature terminal to said opposite coil terminal whereby the current in said coil is reversed and said coil deenergizecl at a rate dependent upon the rate of rise of the countervoltage of said motor, and a resistor in said connection and in the connection from said coil terminal to said first motor resistor.

5. A control system comprising in combination an electric motor, a plurality of current limiting resistors connected in the armature circuit of said motor between one terminal of said armature and one side of a supply source, means for shunting one of said resistors, means for controlling the shunting of another of said resistors an interval of time after the shunting of said first resistor comprising an electromagnetic switching device having an energizing coil connected across said first resistor, and means for varying said time interval in accordance With said countervoltage comprising a connection between one terminal of said coil and the opposite terminal of said armature.

6. A control system comprising in combination an electric motor having connections to a source of power, a plurality of resistors included in said connections between one terminal of the armature and one side of the source, means for shunting one of said resistors, means responsive to the shunting of said resistor for controlling the shunting of a second resistor a time interval after the shunting of said first resistor comprising an electromagnetic switching device having an operating coil, said coil having one terminal connected to said armature terminal and having its opposite terminal connected to a point between said first and second resistors and means for varying said time interval in accordance with the rate of variation of said counter voltage comprising a relatively high resistance connection from said opposite coil terminal to the other terminal of said armature.

'7. A control system comprising in combination an electric motor having a line terminal connected to one side of a supply source and a second terminal connected to the opposite side of said source, a plurality of resistors included in the connections between said second terminal and said opposite side of said source, means for shunting the resistor nearest said armature, a contactor for shunting said resistor, means responsive to the shunting of said resistor for controlling the shunting of a second of said resistors a time interval after the shunting of said first resistor comprising an inductive time element electromagnetic switching device having an operating coil connected to be responsive to the voltage drop across one of said resistors, one terminal of said coil being connected to said second armature terminal, and means for varying said time interval comprising a relatively high resistance connection from the opposite terminal of said coil to the line terminal of said armature.

8. A control system comprising an electric motor having a line terminal connected to one side of a supply source and a second terminal connected to the other side of said source, a plurality of resistors included in the connection between said second terminal and the opposite side of said source, means for shunting a first of said resistors, means responsive to the shunting of said first resistor for controlling the shunt ing of a second of said resistors after an interval of time comprising an inductive time delay relay having an operating coil connected to be responsive to the voltage drop across said first resistor, and means for varying said time interval in accordance with the countervoltage of said motor comprising a resistor connected between one terminal of said coil and a point on said first resistor remote from said armature and a resistor connected between said coil terminal and said line terminal of said motor.

9. A motor starter for a motor comprising connections from a line terminal to one side of a supply source, connections including a plurality of resistors from the opposite terminal to the opposite side of said source, the resistor nearest the armature serving to limit the armature current during plugging operation, means for shunting said plugging resistor, a plurality of contactors, one for shunting each of the other resistors, means responsive to shunting of each of said resistors for controlling one of said contactors to shunt another of said resistors after an interval of time comprising a plurality of inductive time delay relays each having an operating coil connected to be responsive to the voltage drop across one of said resistors, and means for diminishing said time interval in accordance with the rate of increase of the countervoltage of said motorcomprising a connection including a resistor of relatively high resistance from one terminal of each of said coils to said motor line terminal to effect reversal of the current in said coils in accordance with the increase of said countervoltage, and a connection including a resistor from each of said coil terminals to said motor circuit resistors.

10. A motor starter comprising an accelerating resistor and a plugging resistor connected between said accelerating resistor and a terminal of the motor armature, a plurality of contactors, one for shunting each of said resistors, a reversing type master switch for actuating one of said contactors to shunt said plugging resistor,

15 means for actuating another of said contactors to shunt said accelerating resistor after an interval of time comprising an inductive time delay relay having an operating coil and connections including a resistor between said coil and plugging resistor whereby said relay responds to the voltage drop across said plugging resistor and means for diminishing said time interval in accordance with the rate of acceleration of said motor comprising a connection from the opposite terminal of said armature to said coil thereby to reverse the current in said coil in accordance with the rise of the countervoltage of said motor and a resistor in said connection.

BENJAMIN W. JONES. 

